1. Field of the Invention
The present invention relates to a motor-operated power steering apparatus having an electric motor for generating assistive steering power in a steering force transmitting system, and more particularly to a motor-operated power steering apparatus capable of controlling the value of a current supplied to the motor according to the temperature of the motor or other components of the steering force transmitting system.
2. Description of the Relevant Art
There are known motor-operated power steering apparatus having an electric motor for generating assistive steering power in a steering force transmitting system in order to lessen the steering efforts of the driver of a motor vehicle. The motor-operated power steering apparatus control the value of a current supplied to the motor according to steering information such as the steering speed, steering angle, and steering force of the steering wheel or information on the speed of the motor vehicle. The assistive steering power generated by the motor, together with a manual steering force applied to the steering wheel by the driver, is transmitted through a gear mechanism of the steering force transmitting system to dirigible wheels.
The motor-operated power steering apparatus has many heating elements in the steering power transmitting system such as the windings and commutator of the motor, the power transistors for driving the motor, and the sliding members of mechanisms such as a speed reducer. The motor-operated power steering apparatus itself is located in the vicinity of an engine which generates heat, and the ambient temperature around the power steering apparatus varies from season to season. The ambient temperature around the motor-driving power transistors or the mechanisms then tends to change widely, and inevitably increases to quite a high temperature level.
The conventional motor-operated power steering apparatus are designed so that their components such as the motor can withstand high-temperature environments. Under certain special conditions, the motor-operated power steering apparatus are required to operate at temperatures higher than the limit temperature of the components.
Japanese Laid-Open Utility Model Publication No. 61-91465, for example, discloses a motor-operated power steering apparatus designed to protect its components against high-temperature environments. The disclosed motor-operated power steering apparatus comprises a temperature detector for detecting the temperature of the motor, means for ascertaining whether a detected signal from the temperature detector is in excess of a preset temperature range, and means for limiting a current supplied to drive the motor when the temperature of the motor exceeds the preset temperature range. When the motor temperature rises beyond the preset temperature range, the current flowing through the motor is limited to protect the motor.
However, at the time the motor temperature goes above the preset temperature range, the motor current is limited irrespective of how the dirigible wheels are steered. Therefore, when the motor temperature is excessively high, the assistive steering power produced by the motor is lowered regardless of the manual steering force applied to the steering force transmitting system, resulting in a poor steering feeling which the driver has.